The customary procedure for drilling an oil and gas well is to anchor a relatively large diameter surface pipe in the earth down through which the bore hole for the well is drilled. A well head is secured to the upper end of the surface pipe. Suspended within the surface pipe and extending down into the bore hole is typically a string of casing pipe and within the casing pipe a string of tubing, the tubing pipe serving as the conduit by which produce liquids and/or gases are conveyed from the producing formation to the earth's surface. A hanger is employed by which the casing string and/or tubing string is suspended from the well head. A sealing system is required to seal the annular space between the interior of the well head and the exterior of the hanger supported within the well head. This invention provides an improved sealing system for this purpose.
Nearly all casing and tubing hanger seals in use today utilize an elastomeric element and elastomeric seals have known limitations. They tend to loose their effectiveness when exposed to heat or to corrosive substances such as hydrocarbons or hydrogen sulphide or to great pressures over a long period of time. The natural aging process of elastomers causes them to harden and become brittle over time and once hardened and brittle they lose their sealing effectiveness and may thereafter permit leakage to occur. In some instances the long exposure of elastomeric seals to high pressures can cause them to flow and creep permitting leakage of gases or liquids to the environment.
In recent years, effort has been made to replace elastomeric tubing or casing hanger seals with more reliable and longer lasting metal to metal seals. The invention described herein is for an improved metal to metal seal to close the annulus between concentric cylindrical surfaces such as the concentric surfaces between the interior cylindrical surface of a well head and the exterior cylindrical surface of a tubing or casing hanger.
For background information of the art to which the present invention relates reference can be made to the following previously issued United States Patents.
U.S. Pat. No. INVENTOR TITLE 4,131,287 Gunderson et al Annular Seal 4,556,224 Le Crossover Seal Assembly 4,588,029 Blizzard Expandable Metal Seal for a Well Tool 4,646,845 Boeker Metal Seal for Wellhead Apparatus 4,665,979 Boehm, Jr. Metal Casing Hanger Seal with Expansion Slots 4,742,874 Gullion Subsea Wellhead Seal Assembly 4,751,965 Cassity Wellhead Seal Assembly 4,823,871 McEver et al Hanger and Seal Assembly 5,044,672 Skeels et al Metal-to-metal Sealing Pipe Swivel Joint 5,110,144 Burton et al Casing Hanger Seal Assembly 5,325,925 Smith et al Sealing Method and Apparatus for Wellheads